London has been battered by 50mph winds that have felled trees and caused travel chaos. Powerful gusts swept across the capital as the Met Office issued a yellow "be aware" weather alert for most of the country.

The Conservatives today announced that they were opposed to the further expansion of Heathrow airport.

In a significant hardening of the party line, shadow transport secretary Theresa Villiers said the case for a third runway had not been made.

She said the Government should look at alternatives such as greater use of high-speed rail links before sanctioning further growth at the airport.

Transport Secretary Ruth Kelly is due to announce soon whether she will give the green light to a third runway, sixth terminal and a massive increase in the number of flights at Heathrow.

Until now the Tories' official policy has been to favour expansion if it meets planning criteria. But Ms Villiers revealed the party had revised its stance and now feels that further growth should be put on hold. However, she stopped short of saying the Conservatives wanted a permanent halt to airport expansion in the South-East.

The shift in policy is likely to be welcomed by green campaigners. But Ms Villiers risked the anger of business leaders in London who say Heathrow is vital to the continued success of the capital's economy.

She said: "The Government is set on building a third runway regardless of whether key economic and environmental questions are answered. However, they have failed to make the case."

She also questioned the thoroughness of the consultation document outlining expansion. "On page 55 of one of the annexes to the consultation document, we find them casually admitting that they have failed to price in the carbon effects of surface access to the airport...

"And they have included, as an economic benefit, the forecast increase in Air Passenger Duty - despite the fact that much of this will be a transfer from UK citizens to the exchequer, and hence not a benefit for the economy as a whole."

She said the Government should look at a northsouth-high speed rail line linked to Heathrow and the Channel Tunnel as an alternative to expansion.

"We could dispense with the need for hundreds of the flights now clogging up the airport," she said.

She added: "The Government... has given us a consultation document which fails to make the economic case, fails to make the environmental case, and fails to consider the alternatives."

Lord Soley of Future Heathrow, which lobbies for expansion, said: "It is very hard to tell whether the Conservatives would refuse permission for Heathrow to expand. It would be better if they came out clearly against.

"The expansion of Heathrow is vital to London in terms of employment, prosperity and business investment."

Ken Livingstone, Boris Johnson, Brian Paddick and Sian Berry The Conservative Party Liberal Democrats The 2M group of London councils representing two million people Greenpeace National Trust Most London MPs, including Justine Greening (Putney) John McDonnell (Hayes and Harlington) and Susan Kramer (Richmond)